The present invention relates to ink jet printing, and in particular to an improved method for positioning dots produced by a continuous ink jet printer.
Continuous ink jet printers are well known in the field of industrial coding and marking, and are widely used for printing information, such as expiry dates, on various types of substrate passing the printer on production lines. As shown in FIG. 1, a jet of ink is broken up into a regular stream of uniform ink drops by an oscillating piezoelectric element. The drops then pass a charging plate which charges individual drops at a selected voltage. The drops then pass through a transverse electric field provided across a pair of deflection plates. Each drop is deflected by an amount which depends on its charge. If the drop is uncharged, it will pass through the deflection plates without deflection. Uncharged and slightly charged drops are collected in a catcher and returned to the ink supply for reuse. A drop following a trajectory that misses the catcher will impinge on the substrate at a point along a line determined by the charge on the drop. Often, each charged drop is interspersed by a guard drop with substantially no charge to decrease electrostatic and aerodynamic interaction between charged drops. As the substrate is moving past the printer, the placement of the drop on the substrate in the direction of motion of the substrate will have a component determined by the time at which the drop is released. The direction of motion of the substrate will hereinafter be referred to as the horizontal direction, and the direction perpendicular to this, in the plane of the substrate will hereinafter be referred to as the vertical direction. These directions are unrelated to the orientation of the substrate and printer in space. If the drops are deflected vertically, the placement of a drop in the vertical and horizontal direction is determined both by the charge on the drop and the position of the substrate.
It is general practice to provide predefined raster patterns, with the matrix for each pattern, customarily representing a character, of a predetermined size. For example, a 5 high by 5 wide matrix representing an image, as shown in FIG. 2A, can be created which represents a whole image such as a character or a portion of an image. A technique for printing these characters or portions of images which has become widely used is disclosed in U.S. Pat. No. 3,298,030 (Lewis et al). A stroke is defined for each column of the matrix and represents a slice of the image. Each usable drop is assigned to each pixel (dot position) in the stroke. If the pixel is a blank pixel, then the drop is not charged and is captured by the catcher to be sent back to the ink supply. If the pixel is to be printed, an appropriate charge is put on the drop so that it is deflected to follow a trajectory that intercepts the substrate at the appropriate position in the column for that stroke. This cycle repeats for all strokes in a character and then starts again for the next character. If the drops are deflected transversely to the direction of travel of the substrate, a set of drops forming a stroke will clearly lie along a diagonal line, as the substrate will move a certain distance between each drop in the stroke. The angular deviation of the line from vertical will increase with the speed of the substrate relative to the drop emission rate. This angular deviation can be counteracted by angling the deflection plates away from the vertical direction by an amount dependent on the expected speed of the substrate. If drops in a stroke are not sequentially allocated to equally spaced positions on the substrate, the points will no longer lie along a straight line. In order to maintain a simple matrix raster pattern, with straight lines in any direction in the matrix mapping onto straight lines on the substrate, it is necessary to print drops in a stroke sequentially with an equal time interval between each stroke. A stroke takes the same time whether it contains one printed drop or five printed drops. Generally, a varying number of extra guard drops are used at the end of each stroke to permit variation in the substrate speed on a stroke by stroke basis.
It is possible to move away from printing characters based on an orthogonal matrix and instead treat each drop individually, deflecting it using a defined range of charge values. Printing on an orthogonal grid is still possible by selecting the charges in the same sequence as in the stroke based method. This is the approach adopted in International Patent Publication WO 97/06009 (Domino Printing Sciences, PLC). However, this technique makes it much more complicated to generate a font, as it becomes more difficult to determine allowable dot positions. Furthermore, for such a system to be useful, the printhead must be angled at tan.sup.-1 [(I+1)/n] where I is the number of guard drops and n is the number of dots in a column if a standard orthogonal matrix is desired. In normal printing, this will always be at least tan.sup.-1 (2/n). This leads to significant distortion if the print substrate accelerates, as, for any particular print speed, the time between successive drops in any particular column is at least doubled.
It is well known that character definition improves with more dot positions in a vertical stroke and more strokes per character. FIGS. 2A and 2B respectively show characters based on 5.times.5 and 7.times.9 matrices. The 7.times.9 matrix clearly yields better defined characters. However, for a constant drop rate determined by the limitations of the hardware, in order to be able to print at all pixel locations in the matrix, the maximum substrate speed will have to be inversely proportional to the number of pixels per character. Thus, character definition cannot be improved without reducing the maximum substrate speed. The smaller the matrix, the faster the speed but the characters become less defined. There is a conflicting need for better defined fonts at higher speeds, which are still formed from a simple orthogonal matrix.
An approach which has been used to improve character definition while maintaining the same stroke rate is described in U.S. Pat. No. 4,631,557 (Fujimoto et al.). While dots are generally printed along a conventional stroke, each dot can optionally be vertically deflected to a different location approximately half a stroke height away. At this time, the printhead will have moved approximately half a stroke width in the direction of travel of the substrate relative to a dot on the previous stroke at the same vertical position. This therefore gives a way of printing dots along a "virtual" stroke horizontally between two successive conventional strokes. A significant disadvantage of this technique is that varying the number of guard drops between strokes will have a significant effect on the continuity of the dots in an interpolated stroke. A further disadvantage of this technique is that it is very difficult to establish allowable dot patterns in a font, as there are two allowable dot positions for any particular drop which are substantially separated on the grid of allowable dot positions.